Conventional digital pulse width modulated (PWM) power supplies work well for ordinary electronic devices that require a voltage source and are able to tolerate large voltage periodic and random deviation (PARD). In contrast, lasers require a current source wherein deviation in this current source manifests itself in the laser light output as a function of the periodic and random deviation of the power supply divided by the dynamic impedance of the laser. These effects on the output light are especially acute with diode lasers.
Digital PWM power supply output voltage is governed by the equation:Vout=D*Vin;  (1)where Vout is the output voltage and Vin is the input voltage. D is the duty cycle of the PWM. In the case of a digital PWM duty cycle is defined by the equation:(n*Clock)/(m*Clock);  (2)where Clock is the time base of the microprocessor. Thus, (m*Clock) is the period of the switching frequency of the power supply and the (n/m) is the duty cycle D. Further, because both n and m are integers, (1/m)*clock is the smallest control increment. In other words, (1/m)*clock is the smallest increment that the pulse width is able to be adjusted based on the clock. This smallest increment is governed by the equation:[n−(n−1)]/m.  (3)For example, in the case of a 12 volt power supply controlled by a microprocessor with a 10 Mhz clock to drive a single 10 watt laser, a single clock cycle change will result in an incremental change of 10−7 seconds. The switch frequency of 100 Khz power supply is 10−5 seconds. From equation (3) the smallest incremental control change is 10−2. Based on this equation, the smallest adjustment a digital PWM laser controller is able to achieve can be calculated. A typical 10 watt diode laser's dynamic impedance is 0.04 ohm and the incremental change in light power is about one watt per one incremental ampere change. Using Ohms law (voltage=current*resistance) to control the output current from 0 to 10 amps 0.4 volts output change is needed. As indicated earlier the smallest incremental adjustment is 0.12 volts. This will result in 10*(0.12/0.4) amps or 3 amps.
Thus, for applications that require regulation better than a 30% increment, an improved method is needed. To reduce the amount of laser PARD, dither has been used to attempt to reduce the smallest incremental step. Dithering means alternately changing the duty cycle. To get 85% duty cycle the microprocessor alternately changes the duty cycle from 90% and 80%. However, the trade off requires the power supply to have a output smoothing filter ten times larger than necessary. This is not a cost effective trade off as a linear PWM IC could be less expensive than a low frequency power filter.